A. Field of the Invention
This invention relates to the field of radio frequency (RF) receivers. More particularly, this invention related to a monolithic integrated circuit (IC) that includes radio frequency (RF) downconverter circuitry and digital signal processing circuitry for a global positioning system (GPS) receiver, for use in a host product controlled by a microcontroller.
B. Description of Related Art
The Global Positioning System (GPS) is a satellite-based radionavigation system developed and operated by the U.S. Department of Defense. GPS allows land, sea and airborne users to constantly determine their three-dimensional position, velocity, and time anywhere in the world with a precision and accuracy far better than other radionavigation systems currently available. The GPS consists of three segments: user, space and control. The user segment consists of individual receivers, processors, and antennas that allow land, sea or airborne operators to receive GPS satellite broadcasts and compute their precise position, velocity and time from the information received from the satellites.
The space segment presently consists of 24 satellites, in orbit around the earth, which continuously broadcast both position and time data to users throughout the world. These satellites are positioned so that, at any given time, a user at any location on the surface of the earth will be able to receive signals from between five and eight satellites.
The control segment consists of five land-based control and monitoring stations located in Colorado Springs (the master control station), Hawaii, Ascension Island, Diego Garcia, and Kwajalein. These stations monitor transmissions from the GPS satellites as well as the operational status of each satellite and its exact position in space. The master control station transmits corrections for the satellite""s position and orbital data back to the satellites. The satellites synchronize their internally stored position and time with the data broadcast by the master control station, and the updated data are reflected in subsequent transmissions to the user""s GPS receiver, resulting in improved prediction accuracy.
The signals transmitted by the GPS satellites are specified in detail in Interface Control Document, ICD-GPS-200, Rev. C, titled xe2x80x9cNavstar GPS Space Segment/Navigation User Interface,xe2x80x9d which was initially released by the U.S. government on Oct. 10, 1993, and then revised on Oct. 13, 1995, and on Sep. 25, 1997 (xe2x80x9cthe GPS specificationxe2x80x9d). The GPS specification is fully incorporated herein by reference.
Briefly stated, GPS satellites transmit GPS signals in two frequency bands, conventionally referred to as the L1 channel and the L2 channel. The L1 channel has a nominal carrier frequency of 1575.42 MHz, and the L2 channel has a nominal carrier frequency of 1227.6 MHz. Due to the relative motion between the satellite and the observer, however, these frequencies may be Doppler shifted up or down by as much as 5 kHz. Most commercial receivers use only the L1 channel, as the L2 channel is used primarily for military applications. The L1 channel uses two carrier components that are in phase quadrature with each other. Each carrier component is modulated by a separate bit train using binary phase shift keying (BPSK). Each bit train is the modulo-2 sum of 50 bps satellite navigational (xe2x80x9cNAVxe2x80x9d) data and a pseudo-random noise (PN) code. The NAV data includes satellite ephemerides (i.e., satellite position information), timing information, status information, and other data. Two different PN codes are used: the precision (P) code, which may be replaced by the Y-code in some instances, and the coarse/acquisition (C/A) code. Each satellite uses unique PN codes. The C/A code for a given satellite is a particular Gold code with a nominal chipping rate of 1.023 MHz and a length of 1 millisecond. The P-code for a satellite has a nominal chipping rate of 10.23 MHz and a length of 7 days. Accordingly, the P-code provides the most precision position and time information, whereas the C/A code is used primarily for acquisition of the P-code.
GPS receivers perform a number of different processing operations on the GPS signals that they receive in order to obtain the desired position, velocity, and time (PVT) information. First, GPS receivers typically downconvert the high frequency GPS satellite signals to an intermediate frequency (IF) signal. The radio frequency (RF) downconversion process is conventionally accomplished using one or more superheterodyne stages, in which the GPS signal is mixed with a local oscillator (LO) signal. In addition to one or more mixers, the RF downconversion section of GPS receiver typically includes one or more amplifiers and bandpass filters.
Second, the IF signal is typically passed to a digital signal processing section (DSP) section of the GPS receiver. In the DSP section, the IF signal is digitized and the in-phase (I) and quadrature (Q) components are separated out and demodulated. The DSP then processes the carrier-free I and Q components to xe2x80x9cacquirexe2x80x9d the GPS signals from one or more GPS satellites and, once the signals are acquired, to xe2x80x9ctrackxe2x80x9d the GPS signals.
In the acquisition step, a GPS receiver xe2x80x9clocks onxe2x80x9d to the signal transmitted by a particular GPS satellite by determining: (1) the PN codes used by the satellite; (2) the code phase of each PN code, i.e., which xe2x80x9cchipxe2x80x9d in the entire PN code sequence the satellite""s signal currently is at; (3) the Doppler shift of the satellite""s signal; and (4) the time delay between the PN codes from the satellite and a reference used by the GPS receiver. GPS receivers typically make these determinations using correlation methods. Specifically, the GPS receiver xe2x80x9ccorrelatesxe2x80x9d the carrier-free I and Q components with a xe2x80x9creplicaxe2x80x9d PN code that corresponds to the PN code of a GPS satellite that may be in view. The GPS receiver then integrates or low-pass filters the correlated signal to obtain a correlator output. The GPS receiver adjusts the various parameters of the xe2x80x9creplicaxe2x80x9d PN code, such as the particular PN code used, the PN code phase, the PN code time delay, and the Doppler frequency to maximize the correlator output.
Once the GPS signal from a given satellite is acquired, it is xe2x80x9ctrackedxe2x80x9d by adjusting one or more parameters, such as the timing of the xe2x80x9creplicaxe2x80x9d PN code, so that the GPS signal remains highly correlated with the satellite""s PN code. From the timing of the xe2x80x9creplicaxe2x80x9d PN code, relative to a local clock, the GPS receiver is able to calculate a pseudorange, which represents the distance between the GPS receiver and the satellite.
The GPS receiver also obtains the NAV data from the correlator output, from which the GPS receiver obtains the satellite ephemerides and timing information. By combining the pseudorange and NAV data from at least four GPS satellites, the GPS receiver is able to compute its three-dimensional location and to determine the correct time. GPS receivers may obtain this information from multiple satellites by acquiring and tracking signals from several different GPS satellites in succession. However, many GPS receivers are provided with multiple signal processing channels, with each channel corresponding to a particular GPS satellite, so that the GPS can process GPS signals from several GPS satellites at once.
The number of products using GPS information has rapidly increased in recent years. GPS components are now found in numerous products, including cellular telephones, pagers, and personal digital assistants (PDAs). Because these devices require microchip-size components, there is a need for simple, cost-effective miniaturization of navigation receivers for consumer use.
A number of different methods for adding GPS functionality to a product exist. In one such approach, the GPS receiver added to the product includes a radio frequency (RF) downconverter integrated circuit (IC), a signal processing application specific integrated circuit (ASIC), a signal processing memory component, and a microprocessor. This approach is used in the UT, GT, and SL Oncore receivers of Motorola, Inc. A disadvantage with this approach, however, is that the addition of these several integrated circuits to a product can be prohibitive, both in terms of cost and the space available in the product. The addition of the GPS microprocessor also consumes a significant amount power.
In another approach, the GPS receiver added to the product includes an RF downconverter IC and a complex IC that includes a signal processing element, signal processing memory, and a GPS microprocessor. This approach is used in the M12 Oncore receiver of Motorola, Inc. However, the complex IC has a large footprint, a high pin count, and tends to consume a great deal of power. This can make the complex IC difficult to incorporate into many types of products. The complex IC can also be costly to produce.
Another disadvantage with both approaches is that the RF downconverter IC requires its own crystal reference, and the addition of this component further significantly increases the cost, size, and power consumption of the GPS receiver.
Accordingly, there is a need in the art to provide means for adding GPS functionality to products, so that the GPS components take up less space, consume less power, and are less costly than existing approaches for adding GPS functionality to products.
In a first principal aspect, the present invention provides a receiver for a product that is controlled by a microcontroller. The receiver comprises a radio frequency (RF) downconverter for downconverting a radio frequency (RF) signal in a first band of frequencies to an intermediate frequency (IF) signal in a second band of frequencies. The RF signal includes at least one target signal. The receiver also comprises a digital signal processor coupled to both the RF downconverter and the microcontroller. The digital signal processor includes a correlator for acquiring and tracking the at least one target signal in response to signals from the microcontroller. The digital signal processor also includes an asynchronous interface for interfacing said correlator with said microcontroller.
In a second principal aspect, the present invention provides a substantially monolithic integrated circuit that comprises radio frequency (RF) downconverter circuitry and a digital signal processor coupled to the RF downconverter circuitry. The RF downconverter circuitry is for downconverting a radio frequency (RF) signal in a first band of frequencies to an intermediate frequency (IF) signal in a second band of frequencies. The RF signal includes at least one target signal. The digital signal processor includes a correlator for acquiring and tracking the at least one target signal.
In a third principal aspect, the present invention provides a method for using a microcontroller to obtain data from a signal. In accordance with the method, a radio frequency (RF) signal in a first band of frequencies is received and downconverted to an intermediate frequency (IF) signal in a second band of frequencies. The IF signal is digitized to provide a digital signal, and the digital signal is correlated with a replica signal to provide output data. An interrupt signal is generated at predetermined time intervals. In response to the interrupt signal, the microcontroller reads the output data through an asynchronous interface.